As shown in FIG. 7, a cathode ray tube 1 used for receiving television broadcast has an envelope which is basically formed by bonding a panel portion 3 for displaying an image and a substantially funnel-shaped funnel portion 2 which comprises a neck portion 5 housing an electron gun 11, a yoke portion 6 for mounting a deflection coil and a body portion 4, along a sealing portion 10. The panel portion 3 comprises a face portion 7 for displaying an image and a skirt portion 8 to be joined with the funnel portion 2. The panel portion 3 and the funnel portion 2 constitute a glass bulb.
In FIG. 7, numeral 12 designates a phosphor layer which emits fluorescence upon irradiation with an electron beam, 14 designates a shadow mask which determines the positions of the phosphors to be irradiated with an electron beam, and 13 designates a stud pin to fix the shadow mask 14 to the inside of the skirt portion 8. A character A designates the tube axis connecting the central axis of the neck portion 5 to the center of the panel portion 3. The face portion of the panel portion provides a substantially rectangular shape which is formed by 4 sides substantially parallel with the long and short axes which intersect the tube axis A at right angles.
The cathode ray tube utilizes such a principle of operation that thermoelectrons are emitted in a high vacuum from the cathode provided in the electron gun; the thermoelectrons are converged into an electron beam; it is accelerated by the application of a high voltage of from about 25 kV to 35 kV between the cathode and the anode; the accelerated electron beam is bombarded to the phosphors to excite them whereby an image is displayed.
On the other hand, the inside of the cathode ray tube is maintained in a high vacuum state. Accordingly, the difference between internal and external pressures of the glass bulb having an asymmetric structure unlike a spherical shell, acts as an external force to produce a vacuum stress whereby a large tensile stress, i.e., a tensile vacuum stress is generated in the circumference of the face portion, an outer surface of the skirt portion of the panel portion, and in an outer surface of the body portion of the funnel portion.
FIG. 8 shows, as an example, a distribution of stress generated along the short and long axes of the glass bulb wherein the solid line represents a vacuum stress generated in a direction along the paper surface; the broken line represents a vacuum stress generated in a direction perpendicular to the paper surface, and figures along the distribution of stress indicate stress values at the respective positions. As understood from FIG. 8, the tensile vacuum stress is generally large on the short axis, it shows the maximum value at an edge of the face portion in the panel portion, and it shows a larger value in the funnel portion in a portion approaching the opened end of the body portion. The tensile vacuum stress will increase as the thickness of the glass is thinner. Accordingly, there is a high possibility of causing a mechanical fracture in a glass bulb having a thin wall thickness and light weight when a flaw is generated in a region in which the maximum stress exists.
In the glass bulb for a cathode ray tube in such state, if a crack is generated, the crack spreads rapidly to release a high internal deformation energy in it to cause the fracture of the glass bulb for a cathode ray tube. Further, in a state that there is a high tensile stress in the outer surface, a delayed fracture takes place due to the atmospheric moisture to thereby decrease reliability. Accordingly, there has been known that on the premise that the cathode ray tube suffers a flaw, the breaking strength of a glass bulb when it suffers a flaw with a #150 emery sheet, is used as the practical strength of the glass bulb. In a case of glass used for an unstrengthened funnel portion, the practical strength is merely about 24 MPa. Conventionally, in consideration of such practical strength, the maximum tensile vacuum stress σVF admissible to the funnel portion has been at most about 10 MPa.
As simple measures to assure the mechanical strength of the glass bulb without strengthening, there has been used a technique that the thickness of the glass bulb is made sufficiently large. As a result, for example, the mass of the glass funnel having a deflection angle of electron beam of 110° and a screen size of 86 cm reaches about 15.5 kg.
On the other hand, a large number of image displaying devices other than the cathode ray tube have been put to practical use in recent years. In comparison of them with the cathode ray tube, the depth and weight have been taken up as big disadvantages of the display device. Accordingly, there is a strong demand to reduce the depth and weight. However, the reduction in the depth of a conventional cathode ray tube makes its structure more asymmetrical to cause a problem that a large tensile vacuum stress which exceeds extremely the breaking strength of the glass is generated in the glass bulb.
In an attempt to make the wall thickness of glass thinner to reduce the weight, the rigidity of the glass is generally reduced to thereby cause an increase of deformation energy. Since the increase of deformation energy will in particular increase the tensile stress, decline in reliability due to decline of safetiness by fracture or a delayed fracture is aggravating. When the wall thickness of the glass is increased, an increase of the stress can be prevented while controlling the deformation energy. However, the weight is inevitably increased as described above.
As a way to reduce the weight of a glass bulb for a cathode ray tube, there has conventionally been practical to form a compressive stress layer in the surface of the glass panel (the panel portion) in about ⅙ as large as the thickness of the glass by using a thermal tempering method as disclosed in U.S. Pat. No. 2,904,067.
However, in a case of the glass funnel (funnel portion), a glass funnel for a cathode ray tube having for instance, a diagonal conjugate diameter of screen of 86 cm and a deflection angle of 110°, the thickness of the glass at the sealing portion with respect to the glass panel exceeds 13 mm as the maximum, while the thickness of the glass at the sealing portion with respect to the neck portion is less than 3 mm as the minimum. Namely, since the maximum wall thickness is more than 4 times as the minimum wall thickness, it is impossible to uniformly quench the glass funnel having an uneven wall thickness distribution. As a result, the strengthening of the glass funnel by a thermal tempering method has not been put to practical use because a large residual tensile stress develops in the surface concurrently with the compressive stress due to an uneven temperature distribution in the surface.
On the other hand, there has been known to reduce the weight by strengthening the surface of the glass bulb by an ion exchanging method. This method is a method to replace specified alkali ions in the glass with larger ions at a temperature lower than an annealing temperature range to thereby form a compressive stress layer in the surface because of the volume increase. The ion exchanging method utilizing thermal diffusion is advantageous in reducing the weight in comparison with the thermal tempering in the points that a relatively large compressive stress can be obtained and an undesired tensile stress is not generated.
Generally, the composition of the glass usable for a glass funnel contains at least 60 mol % of SiO2, as the major component of glass, and from about 7 to 8.5 mol % of PbO in order to increase the X-ray absorptive power. In addition, it contains an oxide of alkali metal such as sodium and potassium in consideration of the necessity of matching the thermal expansion coefficient with that of another material or the necessity of maintaining an appropriate viscosity at a high temperature while the melting ability and moldability of the glass is taken into account.
On the other hand, the glass is required to have a high electric resistance so as to withstand to a high voltage in operation and so as not to cause the dielectric breakdown of the cathode ray tube. For this, two components of sodium and potassium are well balanced, and a high electric resistance is realized due to a mixed alkali effect. For example, as the glass composition usable generally for a glass funnel, a SiO2—Al2O3—PbO—R2O—R′O type (R2O: an alkali metal oxide and R′O: an alkali earth oxide) is adopted. In order to increase the electric resistance, attention should be paid to the content of alkali having a relatively small ionic radius, such as lithium ions, sodium ions, potassium ions or the like which has a high ionic mobility. Usually, it is unnecessary to contain a lithium oxide in the composition of the glass funnel, and a sodium oxide and a potassium oxide are contained in an amount of from about 5 to 9% by molar percentage.
In case of using an ion exchanging method in which sodium ions are replaced with potassium ions by thermal diffusion, for the above-mentioned glass, the thickness of the compressive stress layer is from 30 μm to 40 μm at the most because the quantities of the sodium oxide and the potassium oxide are already well balanced, and therefore, the mobility of potassium ions is low, and even though it is immersed for 24 hours in a molten liquid of KNO3 of about 450° C. Further, although the compressive stress value has a value of about 80 MPa in the surface, the compressive stress value attenuates exponentially in a thickness direction of the glass, i.e., from the surface of the glass to the inside of it. The depth of a flaw generated in a cathode ray tube, which may suffer during the manufacturing or in a market is about the same as the compressive stress layer. Accordingly, when the compressive stress layer is too thin, there is no effect against a flaw exceeding the thickness of the compressive stress layer. Further, in a state of the cathode ray tube with which a vacuum stress is loaded, the above-mentioned tensile vacuum stress and a strengthened compressive stress are applied together, whereby the effective thickness of the compressive stress layer decreases remarkably, and reliability decreases.
Other than the ion exchange method using thermal diffusion, there has been known ion exchanging using an electric field assisted ion exchanging method (hereinbelow, referred to as the electric field assisting method). JP-A-2001-302278 describes generally such ion exchange method. Namely, a molten liquid of potassium nitrate, a molten liquid of sodium nitrate or a molten liquid obtained by mixing them is prepared; the glass is dipped in the molten liquid; an anode is provided at a surface side of the glass to be subjected to the ion exchanging and a cathode is provided at the opposite surface dipped, and a d.c. voltage is applied to conduct ion exchanging at temperature below the strain point.
The feature of this method is to apply an electric field to increase the mobility of doped ions having a relatively larger ionic radius to replace them with ions having a relatively smaller ionic radius in the glass, whereby a sufficient compressive stress value and a sufficient compressive stress layer depth (thickness) are formed in a short time. However, a funnel-shaped three-dimensional structure having a large volume such as a glass funnel has difficulty in applying uniformly an electric field in an immersed state; a problem during the manufacturing such as a leak current and so on, and a problem concerning characteristics such as warp caused when an outer surface i.e., only a single surface of the glass funnel is strengthened. Accordingly, an effective method for the ion exchanging method of the glass funnel has not been proposed.
It is an object of the present invention to eliminate the disadvantages of conventional techniques to, in particular, reduce the weight of a glass funnel for a flat cathode ray tube. Namely, when a glass funnel having a glass composition in which the contents of a sodium oxide and a potassium oxide are almost balanced is strengthened by ion exchanging method by the conventional ion exchanging method using thermal diffusion, the magnitude of the compressive stress in the compressive stress layer formed by the above-mentioned ion exchanging method attenuates according to an exponential function from its surface to the inside of it even after the dipping for about 24 hours, and the compressive stress becomes zero at about 40 μm at the most.
Further, in a cathode ray tube having a load of vacuum stress, there is a disadvantage that the effective thickness of the compressive stress layer is decreased remarkably in a region where the above-mentioned maximum tensile vacuum stress is generated to thereby decrease reliability. Therefore, there is a strong demand of accomplishment of an ion exchange method which can assure sufficiently the thickness of a compressive stress layer in a relatively short time; does not cause a rapid attenuation of stress in its depth direction, and is suitable for glass for a cathode ray tube having a composition in which an alkali oxide is well balanced.
Further, in an attempt of reducing the weight of a glass funnel or a glass bulb by ion exchanging, it was uncertain as to the limit to the weight reduction in consideration of a stress value, a thickness and a stress distribution of the formed compressive stress layer. Namely, there is a strong demand of accomplishment of a glass funnel of highly reliable and light weight by specifying the relation of the tensile vacuum stress determined by the shape and wall thickness of the glass funnel in consideration of a flaw or the like in a loading state of a vacuum stress after the assembling of a cathode ray tube and a strengthening characteristics obtained by the ion exchanging method.
Further, as disclosed in, for example, JP-B-40-28674, there has been known a method that a salt containing alkali ions used for ion exchange, clay and water are mixed to make paste, and the paste is applied to glass followed by heating it to the temperature at which the salt is molten during which ion exchange is conducted (hereinbelow, referred to as an ion exchanging method using paste). This method is suitable for strengthening partly a limited region of a glass article (hereinbelow, it may be referred to as the partly strengthening) because only the portion coated with the paste can be strengthened by ion exchanging method.
In the conventional ion exchanging method using paste, water is used as a solvent for making the paste because the salt containing alkali ions used for ion exchange is in most cases water-soluble. Accordingly, when the paste is applied to a non-flat glass such as a glass funnel, the salt containing water expands from the portion where the paste is applied. Therefore, the ion exchanging is conducted to a region which is broader than the portion where the paste is actually applied.
Further, the expansion of the water depends on the thickness of the paste, moisture and the surface roughness of the glass, and accordingly, it is difficult to define actually the surface area to be strengthened by ion exchanging method. On the other hand, there has been known the fact that in ion exchanging using the electric field assisting method, the depth of a layer strengthened by ion exchanging method depends on a quantity of electricity per unit surface area. Accordingly, when the surface area to be strengthened by ion exchanging method is not fixed, there causes scattering in the depth of the layer strengthened by ion exchanging method even though the same quantity of electricity is supplied. For ion exchanging partly a limited region of a glass article while the depth and the compressive stress value of the layer strengthened by ion exchanging method are controlled appropriately, a solvent which does not expand after the application is required strongly.